Actually the issue is not with Google Now nor Sony Camera App... The issue is with the MTK soc, and it's not a problem just with Sony devices but it's generally an issue with all mobile phones using non-qualcomm socs. Xperias with MTK chipsets are all affected, Samsung with Exynos CPU are affected and so on. Google warn about this issue in their website, they should put a note or showing a warning when activating the feature.
The issue is that the non qualcomm chipsets do not manage the microphone array like Qualcomm does: in few words the microphone for OK Google (and other similar apps) is shared between apps that need it as a i/o device (like cameras) and apps that need it as a service/sensor (like ok Google) which basically a different processing of audio coming from the microphone before it goes to the bus..
tl;dr: in short Qualcomm can share microphone data between apps and services, Exynos, MTK & Co. Still cannot do that.
